1. Field of the Invention
This invention relates to a photovoltaic element and a method for manufacturing the same, particularly to a photovoltaic element and a manufacturing method of the photovoltaic element which are suitable for semiconductor elements to constitute a solar battery.
2. Related Art Statement
Attention has been paid to a thin film solar battery formed by a vapor phase epitaxy method, and various research and development are carried out for the thin film solar battery. Generally, the thin film solar battery is composed of a photovoltaic element in which a transparent conductive film, a first conduction-type semiconductor layer, an intrinsic semiconductor layer and a second conduction-type semiconductor layer are stacked on a transparent substrate in turn.
FIG. 1 is a structural view showing a conventional photovoltaic element.
In a photovoltaic element 10 depicted in FIG. 1, a transparent conductive film 2, a p-type semiconductor layer 3, an intrinsic semiconductor layer 4, a n-type semiconductor layer 5 and a backside electrode 6 are stacked on a transparent substrate 1.
The transparent substrate 1 is composed of a glass substrate or a resin film made of polyethylene naphthalate (PEN), polyethersulfone (PES), polyethylene terephthalate (PET) or the like.
The transparent conductive film 2 is formed, of tin oxide, ITO, ZnO or the like, in a thickness of about 1 xcexcm or below by sputtering or firing.
The p-type semiconductor layer 3, the intrinsic semiconductor layer 4 and the n-type semiconductor layer 5 are formed in a thickness of about 1 xcexcm or below by plasma CVD, etc. These semiconductor layers include a Si semiconductor material as a base matrix. The p-type semiconductor layer also includes a dopant such as B, and the n-type semiconductor layer also includes a dopant such as P.
The backside electrode 6 is formed, of a metallic material such as Al, Ag, or Ti, in a thickness of about 100 xcexcm or below by sputtering or evaporation.
However, the photovoltaic element 10 has the transparent conductive film 2, on which the above semiconductor layers are formed, and therefore, has a lower open circuit voltage (Voc) than that of a photovoltaic element having a metallic electrode on which the above semiconductor layers are formed.
It is an object of the present invention to repress the degradation of the open circuit voltage (Voc) in the photovoltaic element including the substrate, the transparent conductive film, the first conduction-type semiconductor layer, the intrinsic semiconductor layer and the second conduction-type, different from the above first conduction-type, semiconductor layer.
For achieving the above object, this invention relates to a photo-voltaic element including a substrate, a transparent conductive film provided on the substrate, an intermediate layer, provided so as to cover the transparent conductive film, made in a hydrogen gas atmosphere of 15 volume % or below hydrogen concentration, a first conduction-type semiconductor layer provided on the intermediate layer, an intrinsic semiconductor layer provided on the first conduction-type semiconductor and a second conduction-type, different from the first conduction-type, semiconductor layer provided on the intrinsic semiconductor layer.
Moreover, this invention also relates to a method for manufacturing a photovoltaic element comprising the steps of forming a transparent conductive film on a substrate, forming an intermediate layer so as to cover the transparent conductive film in a hydrogen gas atmosphere of 15 volume % or below hydrogen concentration, and forming, on the intermediate layer, a first conduction-type semiconductor layer, an intrinsic layer and a second conduction-type, different from the first conduction-type, semiconductor layer in turn.
The present inventors have intensely studied the causes that the open circuit voltage (Voc) of the photovoltaic element having the semiconductor layers on the transparent conductive film is smaller than that of the photovoltaic element having the same semiconductor layers on the metallic electrode. As a result, they have considered the cause as follows.
Spear et al. found out by chance that the addition of a small amount of phosphor element or boron element to an amorphous silicon incorporating hydrogen elements changes the properties of the amorphous silicon drastically. Ever since, the silicon semiconductor is made by plasma CVD using a raw material gas such as silane gas and a hydrogen gas for realizing the various properties thereof. Moreover, more amount of hydrogen than the requisite amount for forming the amorphous silicon semiconductor is applied to a film-forming atmosphere, thereby to micronize the crystal grains of the amorphous silicon semiconductor.
Then, it is required in a semiconductor manufacturing process to supply relatively large amount of hydrogen to form much hydrogen plasma, and to resolve and deposit a raw material gas such as silane gas by the hydrogen plasma. Therefore, the hydrogen radical elements incorporated in the hydrogen plasma may reduce the transparent conductive film, and separate metallic elements such as indium elements or zinc elements. Then, the separated metallic elements may exist on the boundaries between the transparent conductive film and the p-type semiconductor layer, and degrade the open circuit voltage (Voc) through the deterioration of the boundary condition.
Therefore, the present inventors have made an attempt to make the transparent conductive film of an oxide material mainly incorporating plasma-proof zinc oxide or cover the transparent conductive film having a smaller resistance with the plasma-proof transparent conductive film. Concretely, a thin zinc oxide film, having a smaller conductivity and reduction sensitivity, is formed on the transparent conductive film made of tin oxide or ITO, and then, the semiconductor layers are formed on the thin zinc oxide film.
However, the above attempt can not repress the reduction of the transparent conductive film. Moreover, the thin zinc oxide film is formed at room temperature by a cheap sputtering apparatus because a high temperature sputtering requires an expensive apparatus. Therefore, the thin zinc oxide film tends to have an amorphous structure, so suffers from the reduction thereof with comparison to the crystalline zinc oxide.
Moreover, it is desired to use a flexible polymer film as the substrate because the polymer film can be produced on a large scale. However, since the polymer film is vulnerable to heating, the transparent conductive film is required to be formed at low temperature. Therefore, the transparent conductive film results in having an amorphous structure.
Furthermore, the present inventors made an attempt to form the semiconductor films on the transparent conductive film by inert gas plasma instead of hydrogen gas plasma for preventing the reduction of the transparent conductive film. In this case, however, the photovoltaic element has only a small open circuit voltage (Voc). Although the reason is unclear, it is considered as the number of the dangling bond of the p-type semiconductor layer on the transparent conductive film increases.
Therefore, the inventors paid attention to the layer structure of the photovoltaic element instead of the manufacturing method for the photovoltaic element.
As a result, the inventors found out that by forming an intermediate layer between the transparent conductive film and the p-type semiconductor layer under a hydrogen concentration atmosphere of 15 volume % or below so as to cover the transparent conductive film, the degradation of the open circuit voltage (Voc) can be repressed. That is, it is considered that the intermediate layer repress the reduction of the transparent conductive film.
In a preferred embodiment of the photovoltaic element of the present invention, an interfacial layer is formed between the first conduction-type semiconductor layer and the intrinsic semiconductor layer. The interfacial layer may improve the boundary condition between the first conduction-type semiconductor layer and the intrinsic semiconductor layer, so that the open circuit voltage (Voc) of the photovoltaic element can be enhanced.
In the case of manufacturing the photovoltaic element having the interfacial layer, the above manufacturing method further includes the steps of forming the interfacial layer between the first conduction-type semiconductor layer on the first conduction-type semiconductor layer and forming the intrinsic semiconductor layer on the interfacial layer.